The invention relates to implantable articles and methods for implanting such articles. More particularly, the invention relates to a bone prosthesis and a method for implanting the same.
There are known to exist many designs for and methods for implanting implantable articles, such as bone prostheses. Such bone prostheses include components of artificial joints, such as elbows, hips, knees and shoulders. An important consideration in the design and implanting of virtually any implantable bone prosthesis is that the bone have adequate fixation when implanted within the body.
Earlier designs of implantable articles relied upon the use of cement, such as polymethylmethacrylate (PMMA) to anchor the implant. The use of such implants can have some advantages, such as providing a fixation that does not develop free play or does not lead to erosion of joining faces postoperatively. However, the current trend is to use the cement to a lesser extent because of its tendency to lose adhesive properties over time. There is also a possibility that cement contributes to wear debris within a joint.
Recently, implantable bone prostheses have been designed to encourage the growth of hard bone tissue around the implant. Such implants are often implanted without cement and the bone grows around surface irregularities, for example, porous structures on the implant.
One such implantable prosthesis is a shoulder prosthesis. During the lifetime of a patient, it may be necessary to replace the natural humeral head and associated glenoid cavity with a prosthesis. Such a shoulder replacement procedure may be necessary to be performed on a patient as a result of, for example, diseases such asosteoarthritis and rheumatoid arthritis, or trauma.
Most shoulder replacement surgeries today involve the implantation of a total shoulder prosthesis. In a total shoulder replacement procedure, a humeral component having a head portion is utilized to replace the natural head portion of the upper arm bone or humerus. The humeral component typically has an elongated intramedullary stem which is utilized to secure the humeral component to the patient's humerus. In such a total shoulder replacement procedure, the natural glenoid surface of the scapula is restructured or otherwise replaced with a glenoid component that provides a bearing surface for the head portion of the humeral component.
With the average age of patients requiring shoulder arthroplasty surgery decreasing, orthopaedic implant manufacturers are developing “bone-sparing” implants for the initial treatment of the degenerative arthritis. While bone-sparing implants for the treatment hip and knee arthroplasty are becoming quite common, bone-sparing shoulder arthroplasty techniques and prostheses are also being developed.
Shoulder surface replacement prostheses are being developed to replace the articulating surface of the proximal humerus with a minimal bone resection and minimal disruption of the metaphysis and the diaphysis. Current designs use a semi-spherical articular dome with a small stem for rotational stability. The under surface of the articular head is also semi-spherical and meets with a spherically machined humeral head.
Typically, however, arthritis of the gleno-humeral joint causes flattening of the humeral head with a large medial osteophyte. The flat humeral head can cause voids in the bone under the prosthesis resulting in limited contact between the prosthesis and the resected bone and may limit the load transfer capability between the prosthesis and the humerus.
Referring now to FIG. 2, a healthy long bone or, in the form of, for example, a humerus 1 is shown. The humerus 1 includes a head 2 on the proximal end of the humerus 1. The head 2 of a healthy humerus has an arculate outer periphery. The arcuate outer periphery is generally hemispherical and meets with a concave glenoid cavity 3.
Referring now to FIG. 3, a diseased humerus 4 is shown. The diseased humerus 4 includes a head 5. The head 5 is flattened as shown in FIG. 3. The humerus 4 also has developed a large medial osteophyte 7.
Referring now to FIG. 4, a prior art prosthesis 8 is shown in position on the head 5 of diseased humerus 4. The head 5 includes a flattened humeral head area or bony defect 9 which leads to a void 6 between the prosthesis 8 and the bony defect 9.
Due to variations in the size and bone configuration of patients, as well as, to the variations in the progress of the diseased long bone and the resultant amount of flattening of the humeral head, a wide variety of sizes and shapes of prostheses are necessary to have a proper fitting prosthesis for most patients.
So that a surgeon may determine and have available the proper prosthesis for a patient, preoperatively, the surgeon may use radiographic techniques such as x-rays to obtain an image of the bone and from that image select the appropriate size prosthesis.
Typically, the surgeon will have available at the operating room the predetermined prosthesis and perhaps the next available size or two both smaller and larger. To verify that the preselected prosthesis is best suited for the patient, orthopaedic prosthesis manufacturers have developed non-implanting substitute prostheses or trials that have the same dimensions as the respective implants and are used to replicate a prosthesis. After the surfaces and cavities are prepared for receiving the prosthesis, the trial is implanted. The arm is then moved through the normal range of motion to determine the appropriateness of the trial and the resulting prosthesis. Such a procedure is called a trial reduction.
Sometimes, the trial reduction will determine that the preselected prosthesis is not optimum for the patient. A larger and sometimes a smaller prosthesis will be more optimum than that preselected. Trials are available with proportional dimensions as they become larger. Thus, a conservative or bone sparing humeral head prosthesis for a smaller articulating surface may also have a smaller stem.
The surgeon first prepares the cavity to receive the stem and the surface to receive the head. If a prosthesis smaller than that originally predicted is required for the patient, the resulting smaller prosthesis and a similarly sized smaller prosthetic trial needs to be placed in the patient. Such a smaller trial does not fit snugly in the prepared cavity. Thus, bone graft needs to be placed in the cavity and the prosthesis placed between the bone graft material and the bone. Such a procedure takes added time in the operating room, causes greater expense and may cause error in positioning, as well as, may not optimize the fixation of the prosthesis.
Referring now to FIG. 6, the prior art prosthesis 8 is shown on the head 5 of the diseased humerus 4. Stem 10 of the prosthesis 8 extends from body 11 of the prosthesis 8. As can be seen, the prosthesis 8 is, for example, a smaller prosthesis than the prosthesis that the head was prepared to receive. A smaller prosthesis was determined to be required after the initial trial reduction with the larger prosthesis. Therefore, a prepared tapered cavity 14 into which the stem 10 is to fit is much larger than the stem 10. Since the cavity 14 was prepared for a larger prosthesis with a correspondingly larger stem, bone graft material 15 is required within the cavity 14 to provide for a secure fitting of the stem 10 to the humerus 4. It should be appreciated that because of the placing of the bone graft and the difference in size between the cavity 14 and the stem 10, the stem 10 and the corresponding prosthesis 8 may be misplaced from its ideal central location in the direction of arrow 16 or rotationally in the direction of arrow 18.